Respiratory hemoproteins such as myoglobin, hemoglobin, and cytochromes from various biological sources are modified by chenical exchange of the prosthetic groups and selective modification of the apoprotein moieties. Effects of such modifications on the electronic state, biological activity, and molecular structure of the hemoproteins are characterized by various physical and chemical techniques. Methods used are, automatic oxygenation measurements, S-, X-, and K-band EPR, proton- and phosphorus NMR, Mossbauer absorption, resonance Raman, and fluorescence spectroscopy.